Photodynamic therapy using porphyrins and related compounds has, by now, a fairly long history. Early work, in the 1940s, demonstrated that porphyrin could be caused to fluoresce in irradiated tumor tissue. The porphyrins appeared to accumulate in these tissues, and were capable of absorbing light in situ, providing a means to detect the tumor by the location of the fluorescence. A widely used preparation in the early stages of photodynamic treatment both for detection and for therapy was a crude derivative of hematoporphyrin, also called hematoporphyrin derivative, HpD, or Lipson derivative prepared as described by Lipson and coworkers in J Natl Cancer Inst (1961) 26:1-8. Considerable work has been done using this preparation, and Dougherty and coworkers reported the use of this derivative in treatment of malignancy (Cancer Res (1978) 38:2628-2635; J Natl Cancer Inst (1979) 62:231-237).
Dougherty and coworkers prepared a more effective form of the hematoporphyrin derivative which comprises a portion of HpD having an aggregate weight &gt;10 kd. This form of the drug useful in photodynamic therapy is the subject of U.S. Pat. No. 4,649,151, is commercially available, and is in clinical trials.
The general principles of the use of light-absorbing compounds, especially those related to porphyrins, has been well established as a treatment for tumors when administered systemically. The differential ability of these preparations to destroy tumor, as opposed to normal tissue, is due to the homing effect of these preparations to the objectionable cells. (See, for example, Dougherty, T. J., et al., "Cancer: Principles and Practice of Oncology" (1982), V. T. de Vita, Jr., et al., eds., pp. 1836-1844.) Efforts have been made to improve the homing ability by conjugating hematoporphyrin derivative to antibodies. (See, for example, Mew, D., et al., J Immunol (1983) 130:1473-1477.) The mechanism of these drugs in killing cells seems to involve the formation of singlet oxygen upon irradiation (Weishaupt, K. R., et al., Cancer Research (1976) pp. 2326-2329).
The use of hematoporphyrin derivative or its active components in the treatment of skin diseases using topical administration has also been described in U.S. Pat. No. 4,753,958. In addition, the drugs have been used to sterilize biological samples containing infectious organisms such as bacteria and virus (Matthews, J. L., et al., Transfusion (1988) 28:81-83). Various other photosensitizing compounds have also been used for this purpose, as set forth, for example, in U.S. Pat. No. 4,727,027.
In general, the methods to use radiation sensitizers of a variety of structures to selectively impair the functioning of biological substrates both in vivo and in vitro are understood in the art. The compounds useful in these procedures must have a differential affinity for the target biological substrate to be impaired or destroyed and must be capable of absorbing light so that the irradiated drug becomes activated in a manner so as to have a deleterious effect on the adjacent compositions and materials.
Because it is always desirable to optimize the performance of therapeutics and diagnostics, variations on the porphyrin drugs traditionally used in treatment and diagnosis have been sought. A number of general classes of photosensitizers have been suggested including phthalocyanines, psoralen-related compounds, and multicyclic compounds with resonant systems in general. Most similar to the compounds disclosed herein are various pheophorbide derivatives whose use in photodynamic therapy has been described in EPO Application 220686 to Nihon Metaphysics Company; ethylene diamine derivatives of pheophorbide for this purpose described in Japanese Application J85/000981 to Tama Seikayaku, K. K., and Japanese Application J88/004805 which is directed to 10-hydroxy pheophorbide-a. In addition, pheophorbide derivatized to a long chain hydrocarbyl group has been disclosed as useful in photodynamic therapy in U.S. Ser. No. 221,804, filed Jul. 20, 1988, assigned to the same assignee and incorporated herein by reference. In addition, Beems, E. M., et al., in Photochemistry and Photobiology (1987) 46:639-643 discloses the use as photosensitizers of two derivatives of bacteriochlorophyll-a --bacteriochloro-phyllin-a in which the exocyclic ring is opened and lacks the phytyl alcohol derivatized in bacteriochlorophyll-a) and bacteriochlorin-a (which is similar to bacteriochlorophyllin but lacks the Mg ion. These authors direct their attention to these derivatives as being advantageous on the grounds of enhanced water solubility as compared to bacteriochloro-phyll-a.
The problem remains to find suitable photosensitizers useful in photodynamic therapy and diagnosis which are optimal for particular targets and particular contexts. It is unlikely whether a single compound or small group of compounds, while generally applicable, would be of maximum benefit in every instance. Thus, the invention provides an additional group of photosensitizing compounds which becomes part of the repertoire of candidates for use in specific therapeutic and diagnostic situations.